Scalar Mixing In Turbulent Flow

Author : Alais Hewes
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (125 download)

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Download or read book Multi-scalar Mixing in Turbulent Coaxial Jets written by Alais Hewes and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "An experimental investigation of multi-scalar mixing in turbulent coaxial jets is presented herein. The first part of this work involves the development of a thermal-anemometry-based probe to simultaneously measure two scalars and velocity in turbulent flows. The work of Hewes (2016) on the simultaneous measurement of velocity and helium concentration is extended, and the necessary and optimal design parameters for thermal-anemometry-based probes capable of making such measurements are refined in the current work. An analysis of measurements from different interference probes reveals that, unlike what has previously been reported in the literature, hot-wires of differing diameters are not required for concurrent velocity and concentration measurements. Furthermore, both theoretical arguments and experiments in flows of varying helium concentration are used to demonstrate that the cold-wire thermometer is effectively insensitive to helium concentration. The temperature field can therefore be measured independently of the velocity or concentration fields. This has important implications for interference probes, since the cold-wire can be used to compensate for temperature effects on these probes, allowing them to be used in non-isothermal flows. Consequently, it is shown that an interference probe can be combined with a cold-wire thermometer to form a novel 3-wire probe to simultaneously measure velocity, helium concentration, and temperature in a turbulent flow. \\ \indentIn the second part of this work, the novel 3-wire probe is used to study the evolution of multiple scalars in turbulent coaxial jets consisting of (i) a center jet containing a mixture of helium and air, (ii) an annular jet containing pure (unheated) air, and (iii) a coflow of (pure) heated air. The flow can be viewed as containing three scalars: $\phi_1$, normalized helium concentration, $\phi_2$, pure, unheated air in the annular jet (which is inferred from measurements of the other two scalars), and $\phi_3$, normalized temperature. Measurements are performed along axis of the jets for three different momentum ratios ($M_2/M_1=0.77$, $M_2/M_1=2.1$, and $M_2/M_1=4.2$). Mean and rms quantities, covariances, velocity-scalar triple moments, probability density functions (PDFs), joint probability density functions (JPDFs), and conditional expectations of the fluctuating velocity are measured. Three distinct regions are identified along the axis of the coaxial jets: (i) the potential core of the center jet, consisting primarily of $\phi_1$, (ii) the inner mixing region, dominated by large fluctuations of anti-correlated $\phi_1$ and $\phi_2$, and (iii) the fully merged region, where the coaxial jets behave similarly to a single jet. In this final region, one can observe that $\phi_1$ and $\phi_3$, and $\phi_2$ and $\phi_3$, become progressively anti-correlated in the downstream direction, whereas $\phi_1$ and $\phi_2$ become progressively more correlated. Increasing the momentum ratio ($M$) initially causes $\phi_1$ and $\phi_2$ to mix more quickly, however, examination of the JPDFs suggest that far downstream, these effects may be reversed. Furthermore, distinct differences are observed between coaxial jets in which $M1$ and those in which $M1$, most likely resulting from the fact that the annular jet is entrained into center jet in the former, whereas the center jet is entrained into annular jet in the latter. Finally, it is worth pointing out that $M$ may have a significant effect on the shapes and distributions of the scalar-scalar JPDFs, even where the correlation coefficients of different cases have converged to similar values. Thus, the need for simultaneous multi-scalar measurements when studying multi-scalar mixing is emphasized"--